Datasheet

8
LT3473/LT3473A
3473f
resulting sum is fed into the positive terminal of the PWM
comparator A2. When this voltage exceeds the level of the
error signal V
C
, the SR latch is reset, turning off the power
switch Q1. The error amplifier sets the peak current level
to keep the output in regulation. If the error amplifier’s
output increases, more current is delivered to the output;
if it decreases, less current is delivered.
The LT3473 includes an internal power Schottky diode and
a PNP transistor, Q2, for output disconnect. Q2 discon-
nects the load from the input during shutdown. The part
also has a power good indication pin, PGOOD. When the
FB voltage reaches within 100mV of the reference voltage,
the comparator A4 turns on Q5, sinking current from
PGOOD pin.
The LT3473 has thermal shutdown feature with threshold
at about 145°C.
Inductor Selection
A 6.8µH inductor is recommended for the LT3473. The
minimum inductor size that may be used in a given appli-
cation depends on required efficiency and output current.
Inductors with low core losses and small DCR (copper
wire resistance) at 1.2MHz are good choices for LT3473
applications. Some inductors in this category with small
size are listed in Table 1. The efficiency comparison of
different inductors is shown in Figure 3.
Capacitor Selection
The small package of ceramic capacitors makes them
suitable for LT3473 applications. X5R and X7R types of
ceramic capacitors are recommended because they retain
their capacitance over wider voltage and temperature
ranges than other types such as Y5V or Z5U. A 4.7µF input
capacitor, a 0.47µF output capacitor and a 2.2µF capacitor
bypassing output disconnect PNP are sufficient for most
LT3473 applications.
Table 2. Recommended Ceramic Capacitor Manufacturers
MANUFACTURER TELEPHONE URL
Taiyo Yuden 408-573-4150 www.t-yuden.com
AVX 843-448-9411 www.avxcorp.com
Murata 814-237-1431 www.murata.com
Kemet 408-986-0424 www.kemet.com
APPLICATIO S I FOR ATIO
WUUU
Table 1. Recommended Inductors
DCR CURRENT DIMENSION
PART (mΩ) RATING (A) (mm) MANUFACTURER
DO1605T-682 200 1.1 5.4 × 4.2 × 1.8 Coilcraft
ME3220-682 270 1.0 3.2 × 2.5 × 2.0 800-322-2645
MSS6122-682 100 1.45 6.1 × 6.1 × 2.2
www.coilcraft.com
MSS5131-682 60 1.05 5.1 × 5.1 × 3.1
LQH55DN6R8 74 2.0 5.7 × 5.0 × 4.7 Murata
814-237-1431
www.murata.com
CDRH5D18-6R2 71 1.4 5.7 × 5.7 × 2.0 Sumida
CDRH4D28-6R8 81 1.12 4.7 × 4.7 × 3.0 847-956-0666
CDRH5D28-6R2 33 1.8 5.7 × 5.7 × 3.0
www.sumida.com
CRD53-4R7 74 1.68 6.0 × 5.2 × 3.2
A918CY-6R2M 62 1.49 6.0 × 6.0 × 2.0 Toko
(TYPE D62LCB) 408-432-8281
A915AY-6R8M 68 1.51 5.0 × 5.0 × 3.0
www.tokoam.com
(TYPE D53LC)
LEAD CURRENT l
O
(mA)
0
EFFICIENCY (%)
70
75
80
80
3473 F03
65
60
55
20
40
60
100
TOKO A915AY-6R8M
SUMIDA CDRH4D28-6R8
SUMIDA CDRH5D18-6R2
COILCRAFT ME3220-682
COILCRAFT MSS5131-682
V
IN
= 3.6V
V
OUT
= 20V
Figure 3. Efficiency Comparison of Different Inductors